1. Field of the Invention
This invention relates to an improved phase grating focusing plate which has a number of fine unit patterns consisting of projections and/or recessions on a surface of a substrate such that the unit patterns are arranged regularly in a regular square configuration or in a regular triangle configuration.
2. Description of the Prior Art
Viewfinders in various types of cameras have utilized diffraction grating plates to permit focusing. Generally, a centralized image pattern is progressively brought into focus by adjustment of the objective lens system. When the focal point is coincident with the diffraction plate, the image appears clearly in the viewfinder.
Several problems commonly occur in the focusing of optical instruments, such as cameras. For example, in observing the image focused on a focusing plate via an optical focusing system, the wider the angle of diffusion on the focusing plate, the easier it is for the operator to visualize the defocused state and to bring the image into focus. The increased angle, however, diminishes the quantity of light striking the operator's eye and the brightness of the visual field of the viewfinder. On the contrary, the smaller the angle of diffusion on the focusing plate, the brighter the visual field but the more difficult it is to focus an image. In other words, the ease of focusing and the brightness of the visual field are conflicting conditions.
A conventional focusing plate consisting of a ground glass plate generally exhibits the characteristics that the intensity of diffusion light is high at the angle of diffusion of 0.degree. (optical axis) and decreases suddenly with any increase in the angle of diffusion. As a result, it can be observed that a defocused image from a point source is very sharp at its center and degrades gradually away from its center, with resulting difficulties in focusing. Secondly, in the case of projections and recessions irregularly distributed on the glass substrate with slanted upper surfaces forming an acute angle with respect to the optical axis, refracted light from these slanted surfaces moves away from the operator's eye so that grain-shaped black spots are observed by the operator's eye and the overall image quality of the viewfinder screen deteriorates.
Many focusing plates of a phase grating type have been suggested, such as set forth in Japanese Patent Laid-Open Publications No. 47/27531, 52/154629, 52/145221, 53/31127, 55/9568 and German Patent Laid-Open Publication No. 2,946,005.
According to Laid-Open Publication No. 52/154629, a space filter with a specific phase difference .pi. is used such that "0"th diffraction light is extremely faint and ".+-.1"th diffraction efficiency is at most 40%. In this case, the use of a diffraction grating renders it difficult to focus on objects in any direction other than a specific one because of its limited directionality. The focusing plates disclosed in Laid-Open Publications No. 47/27531 and 42/145221 also suffer from the above-described disadvantages.
In contrast, a phase grating type focusing plate disclosed in Publication No. 53/31127 is universal and has two-dimensional light diffusion characteristics. However, there is not any teaching as to particular kinds of light diffusion characteristics that are best to facilitate focusing.
With respect to Japanese Patent Laid-Open Publications Nos. 55/9568 and 53/31127 and German Patent Laid-Open Publication No. 2,946,005, each unit pattern of these known focusing plates is provided with either a single central cylindrical projection or a recession or a single set of alternatively arranged ring-shaped concentric projections and recessions. The known focusing plates having such unit patterns suffer from a disadvantage in that freedom or flexibility of design for obtaining desired diffraction characteristics is poor, although it is possible to modulate diffraction characteristics by changing the level difference between the projections and the recessions (i.e., the phase difference .theta. of a light due to the level difference), and by changing the radius of the cylindrical projection or recession or the combination of the radii of the ring-shaped concentric projections and recessions (i.e., the ratio .gamma. of the total areas of the projections or recessions to the overall area of the unit pattern). In addition, it is another disadvantage of the known focusing plates that even the slightest error in the radius of the cylindrical projection or recession or in the combination of the radii of the ring-shaped concentric projections and recessions, which may occur upon production, affects the diffraction characteristics to considerable extent.
It is to be noted that focusing plates of the described type having unit patterns of different configurations are schematically illustrated in FIGS. 4 and 7 of Japanese Laid Open Publication No. 55/9568. Each of these unit patterns is provided with asymmetrically arranged cylindrical or rectangular parallelopiped projections. The asymmetrical arrangement of such projections in the unit patterns, however, cannot provide defocused images with natural gradation. Accordingly, there is a need in the prior art to provide an optimized focusing plate that provides a bright visual field with improved focusing capabilities while increasing the tolerance of manufacturing errors.